Power transistors having a low ON-resistance RON and a low gate-source and gate-drain capacity CGS and CGD (in total low input capacitance) are required for use in DC/DC converters. According to U.S. Pat. No. 5,973,360 issued to Tihany (the '360 patent) the lower ON-resistance of an SFET3 is achieved by the field plate principle. In particular, the '360 patent discloses a field effect-controllable semiconductor component that includes a semiconductor substrate. A drain zone of a first conductivity type is disposed in the semiconductor substrate and a gate electrode composed of polycrystalline silicon is also disposed in the semiconductor substrate. A source region of a second conductivity type is introduced in the drain zone. The drain zone includes a trench structure that reaches from the surface of the semiconductor substrate into the drain zone. A field plate is in the trench structure and an oxide layer surrounds the field plate. The oxide layer has a thickness that increases in a direction towards the drain zone.
In order to lower the gate-drain capacitance of power MOSFETs, the gate electrode G may be replaced by a field electrode F set at a source potential or another defined potential (see, for example, U.S. Pat. No. 5,283,201 to Tsang), and a gate electrode G′ having a very small overlap region with the drain region. In this arrangement the small CGD dispersion is adjusted by etching of the field electrode. The field plate is wet-chemically defined via the field electrode. This results in a considerable surface ratio of source-gate electrode overlap, which on the one hand defines the CGS and on the other hand, in thin field electrode oxides, also defines the minimal thickness of the gate oxide in the transition region from the thin oxide (MOS channel region) to the thick oxide (field electrode region).
Up to now, the higher CGS has been tolerated for the benefit of the low CGD that can be achieved by this concept. With regard to a gate oxide thinning, a minimal planar oxide thickness has to be defined in order to comply with the service life requirements of the oxide. Consequently, the performance of the transistor with regard to the ON-resistance (in particular RON at a gate voltage VG of 4.5 V) is limited.